This invention relates to hollow fibers useful in separation schemes such as reverse osmosis, ultrafiltration, and gas separations. The hollow fibers of this invention are designed so that ultrathin coatings can be supported on either their external or internal surfaces, and the composite structure operated at high pressures to effect the separations.
The process of ultrafiltration refers to the separation of a dissolved solute from its solvent (which is generally but not necessarily water), through the application of a pressure gradient. In the ultrafiltration operation solvent permeates the barrier in preference to the solute, so that separation is effected. According to the well understood principles of osmosis, the separation of a solute from its solvent by means of a membrane impermeable to one or more of the components of that solution generates an osmotic pressure in opposition to the gradient of concentration. This pressure is proportional to the molar concentration of the non-permeating species according to the Van't Hoff law. When selective permeation occurs as a result of a pressure gradient under circumstances such that the back osmotic pressure is negligible, the process is termed ultrafiltration. If, however, the process is carried out under circumstances where the back osmotic pressure is significant, the process is generally termed "Reverse Osmosis." The distinctions are somewhat arbitrary in the literature, as is evidenced from the definition given here.
The desirability of performing separations by these techniques has been well documented; the economics of such separations are generally a function of the productivity attainable through the separating membrane, and this productivity is a function of both the specific properties of the membranes, and the conditions under which they can be operated continuously. The significant properties of the separating membranes which relate to productivity are:
a. High hydraulic permeability to the solvent.
b. Good chemical stability in the presence of the solvent, solutes, and incidental components present in the solutions, or mixtures, to be treated.
c. High efficiency in rejecting permeation of the solute to be retained.
d. High values for the Young's modulus of the membrane material under service conditions, so that the process can be operated at high pressures without causing creep failure of the material, or collapse of its morphological structure.
Two general approaches have been taken by previous investigators toward providing separation membranes having desirable properties.
One approach has been to provide homogeneous structures, generally manufactured as thin as practical, with morphologies that are so arranged that there exist pore structures which are capable of allowing solvent to permeate, but are too small for the transport of solutes. Structures of this kind can be termed ultrafilters since they operate by a filtration mechanism. The difficulty of preparing narrow distributions of pore sizes which are capable of selectively retarding certain molecular species has led to the restriction of such membranes to separations limited to high molecular weight solutes where the difference in molar size between the solvent and the solute is sufficient. If a comprise is sought to allow the separation of smaller molecules from solvents, the pore size reduction needed is often so great that the resulting permeability of the membrane is less than can be used practically.
A second approach to this problem has been to prepare separation membranes which operate by allowing only the solvent to diffuse through the membrane structure. However, such membranes exhibit such extremely low rates of solvent transport that they are of no practical interest in such diverse applications as desalination, protein purification, solvent recovery, or separation of organic mixtures. Diffusive membranes are desirable in that they offer the greatest selectivity in separation, and it is known that such barriers prepared in ultrathin form (so that the diffusion path is reduced to less than 1,000 Angstroms) have application in a number of processes.
The preparation of composite membranes consisting of an ultrathin layer contiguous with a more porous substrate has been taught by Loeb, using cellulose acetate as the polymer (Adv., Chem., Ser. 38, 117 ), and by Michaels from acrylonitrile, polysulfone, and polyvinyl chloride (U.S. Pat. Nos3,526,588 and 3,615,024). Such integral structures have been demonstrated to consist of a dense microporous surface contiguous with a porous support structure. The latter is prepared with as high a porosity as possible to reduce the hydraulic resistance of the support layer.
In an attempt to overcome the difficulties and limitations of forming such a composite membrane from a single polymer (since the requirements for the barrier layer and its support are often diametrically opposed), Riley and Lonsdale ("Development of Ultrathin Membranes," R.L. Riley, H.K. Lonsdale, I.D. Lagrange and C.R. Lyons, O.S.W. R & D Report #386 PB 207036, U.S. Government Printing Office) have disclosed the formation of such membranes by a two-step process. In this procedure the solute rejecting diffusive layer is placed on a porous support which has been preformed; this permits independent consideration for the material needs of each layer. This coating procedure allows the selection of porous supports which are resistant to deformation at high pressures, and at the same time it does not require that the rejecting layer be made from the same material. Consequently, water absorbing surface coatings can be placed as an ultrathin film on a non-water swelling support which will not deform as a result of high pressures in the presence of water.
The requirements for preparation of porous supports for such bicomponent structures are very strict, and have not been met except in restricted instances. The support must have a microstructure which is fine enough to support ultrathin films of the barrier coating without allowing the coating to rupture into the supporting pore as pressure is applied. At the same time, the porous support must exhibit a sufficiently high solvent permeability so that the solvent which has permeated the barrier coating is not retarded further. And finally the porous substrate must be so textured that despite these preceding requirements, it can withstand application of high pressures without failing under creep.
The present invention provides a new technique for preparing such microporous support materials in the form of hollow fibers. While hollow fibers have been described before for chemical separations, none are suitable for the demands outlined here. Asymmetric fibers as described by McLain and Mahon (U.S. Pat. No. 3,423,491) have much higher hydraulic resistances than can be used in this application. Fiber prepared according to the teachings of Michaels (U.S. Pat. Nos. 3,526,588 and 3,615,024) have a pore structure which causes collapse of the fiber walls when pressures in excess of 200 psi are applied. And even fibers made from aliphatic and aromatic nylons have been found to undergo creep failure at pressures in excess of 600 psi.
The fibers described in this invention are capable of operation at pressures up to 2,000 psi, and have hydraulic resistances which are useful in combination with selective ultrathin coatings.